Particulate emissions from diesel engines have received considerable attention from governmental regulatory agencies worldwide. Significant research into their health effects has shown that the toxicity impact on human health is much greater than originally perceived. Most of the toxicity of diesel exhaust pollutants is created by VOC compounds and the phenomenon of nano-particle formation and count. Although VOCs and nano-particle count are not regulated yet, measures will be taken to limit their discharge to the atmosphere to the lowest achievable level, which will be the subject of future governmental regulations. Nitrogen oxide is the culprit in the formation of smog and acid rain; sulfur dioxide is a major contributor to acid rain.
Engine technology has drastically advanced over the last ten years. The modern generation of diesel engines is capable of achieving emissions of 0.1 gm/bhp.hr, compared to 0.60 gm/bhp.hr in 1988. Although modern diesel engines are significantly cleaner than older diesel engines, the demand for cleaner exhaust will continue until near-zero emissions are achieved. Currently, USEPA and CARB regulations are targeting particulate emissions down to 0.01 gm/bhp.hr and NOx, emissions down to 0.2 gm/bhp.hr starting with heavy truck vehicles model year 2007. As it stands today it is very difficult, if not impossible, to achieve such target emissions only utilizing engine technology. That leaves the exhaust after-treatment option as a most valid alternative to comply with the regulations.
After-treatment technologies for the capture of diesel particulate-matter and lowering NOx have received considerable attention over the last twenty-five years. Most of these technologies are focused on capturing particulate matter on filtration media such as cordierite ceramic wall-flow filters, ceramic-fibers wound on perforated tubes, and metallic fiber filter media. Such devices are commonly known as particulate traps.
While particulate traps have proven to be effective filtration media with efficiencies that can reach 80-95%, it is necessary to rid the filter media of accumulated soot to bring it back to the initial conditions for another cycle of filtration. This need has led to the development of what is now well known as the “regeneration process”. Although the principles of the regeneration process are simply based on burning the accumulated soot, they are not yet reliable in practical applications. In this regard, regeneration process and particulate traps have severe limitations in real world applications. For example, regeneration must be initiated when filter loading reaches a threshold value beyond which pressure drop across the filter media starts to rapidly increase and would interfere with engine performance operation. From a statistical point of view, the exhaust temperature profiles during diesel truck operation are not sufficiently high to initiate the regeneration process when it is needed. Means were incorporated to facilitate the regeneration process, such as “induced or forced regeneration” in which an external source of heat is employed to raise the temperature of the filter media above the soot ignition temperature to initiate combustion. Alternatively, precious and or base metal catalysts were proposed in the form of a coating on the filter media or as an additive to the diesel fuel. Catalysts can bring the soot ignition temperature down from 620° C. to as low as 320° C., which would enhance the probability of achieving regeneration during engine operation by relying on exhaust temperature profiles, especially at high engine loads. Relying on catalysts to achieve regeneration raised other problems related to catalyst poisoning from sulfur compounds in diesel fuel. This led to the introduction of ultra-low sulfur diesel fuel to ensure durable operability of catalysts. Although the probability of a successful regeneration in real life application has improved over the years, the regeneration problems are not completely eliminated. In the final analysis, complex and expensive hardware having elaborate logics were deployed to work in the harsh exhaust environment, which exacerbated other problems such as reliability and durability in operation.
The most critical limitation associated with the regeneration process in particulate traps relates to reliability in operation, which is a crucial factor, especially in mobile applications. Diesel engine vehicles do not follow a single pattern of driving cycle on the road. Rather, some diesel powered vehicles experience prolonged idling conditions while others operate in congested traffic zones. All these factors render exhaust temperature profiles too low to accomplish regeneration in a passive system. This is true even in the presence of catalysts. As a result, unwarranted problems are created during operation. Although such problems can be typically rectified through “forced regeneration”, the associated active components such as fuel injection in the exhaust, valves, microprocessors, thermocouples, and the like have proven to create extensive maintenance and poor reliability in the harsh exhaust environment. Components at or near an exhaust system must be qualified to high shock loading up to 30 g's as well as thermal shock loading. The reliability of active components in diesel exhaust environment has proven to be poor.
Durability is by far another major challenge for particulate trap systems required to achieve durability of 450,000 miles, as well as maintenance-free intervals of 150,000 miles, according to EPA. Most active components and systems lack the ability to meet such durability requirements, due to unwarranted shock loadings, thermal shock stresses, and other related factors.
NOx control technologies are diversified. Significant control technologies include lean-burn catalysts, plasma-assisted catalysts, adsorbents, selective catalytic reduction, and exhaust gas re-circulations (EGR). Almost all of these known technologies are effective in reducing NOx emission 25% to 90%. However, each technology has certain problems similar to those associated with particulate traps. By far, exhaust gas re-circulation is the most promising technology having a manageable set of problems. In diesel engines, EGR problems include: (1) contamination of exhaust gas with soot, which creates problems in the air intake system of an engine, (2) high exhaust temperatures interfering with engine performance, and (3) insufficient pressure differential to drive the necessary exhaust flow to the engine air intake to maintain proper circulation. These problems have hampered the acceptance of EGR technology in diesel engine applications. Developments led to the evolution of new EGR concepts for diesel engines, such as high-pressure and low-pressure strategies and combinations thereof. Known EGR systems can be complex and employ extensive hardware that also has the potential for poor durability, poor reliability and high fuel penalty.
In summary, the problems of regeneration and EGR adaptation are still eluding researchers despite improvements. In particular, having soot entrapped in a filter media has proven to be a very difficult and elusive task, without clear resolution warranting acceptance in mobile and stationary diesel applications. A clear need for advancement of the pertinent art exists.